In 1921, when he was 39 years of age, Franklin Delano Roosevelt contracted an illness characterized by: fever; protracted symmetric, ascending paralysis; facial paralysis; bladder and bowel dysfunction; numbness; and dysaesthesia. The symptoms gradually resolved except for paralysis of the lower extremities. The diagnosis at the onset of the illness and thereafter was paralytic poliomyelitis. Yet his age and many features of the illness are more consistent with a diagnosis of Guillain-Barré syndrome, an autoimmune polyneuritis. The likelihoods (posterior probabilities) of poliomyelitis and Guillain-Barré syndrome were investigated by Bayesian analysis. Posterior probabilities were calculated by multiplying the prior probability (disease incidence in Roosevelt's age group) by the symptom probability (likelihood of a symptom occurring in a disease). Six of eight posterior probabilities strongly favoured Guillain-Barré syndrome.